Circuit and method for detecting current zero-crossing point and circuit and method for detecting load voltage

ABSTRACT

A circuit and a method for detecting a current zero-crossing point, and a circuit and method for detecting a load voltage are disclosed. The circuit for detecting current zero-crossing point includes: a load power supply circuit, a voltage-dividing resistor, a transistor switch, a zero-crossing detection circuit; the load power supply circuit includes: a load, a diode, and a transformer; one end of a primary winding of the transformer is connected with the operating voltage input terminal, the other end of the primary winding of the transformer is connected with a first end of the transistor switch and a first end of the voltage-dividing resistor, a second end of the voltage-dividing resistor and a second end of the transistor switch are connected with the ground, the load voltage is controlled by the transistor switch.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.15/612,998, filed on Jun. 2, 2017, which published as U.S. PublicationNo. 2017-0310313 A1 on Oct. 26, 2017, which claims the benefit ofChinese Patent Application No. 201410729856.2, filed on Dec. 4, 2014,the contents of which are incorporated herein by reference in theirentireties.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The present disclosure relates to the technical field of circuitdetection, more particularly, to a circuit and method for detectingcurrent zero-crossing point, and a circuit and method for detecting loadvoltage.

Background of the Disclosure

With the rapid development of LED, designs of LED driving circuits areemerging endlessly. However, the current LED driving circuit is complexin terms of designs in functional aspects like output over-load andover-voltage protection. Common LED load detection circuit uses atransformer, which makes the circuit complex, with a large volume andhigh cost.

SUMMARY OF THE DISCLOSURE

The present disclosure provides a circuit and method for detectingcurrent zero-crossing point, and a circuit and method for detecting loadvoltage, which can rapidly obtain load voltage and rapidly determinescurrent zero-crossing point of a diode; the circuit volume is small, andcost is low.

In order to solve the above problem, embodiments of the presentdisclosure provide a circuit for detecting current zero-crossing point,wherein it includes: a load power supply circuit, a voltage-dividingresistor, a transistor switch, a zero-crossing detection circuit; theload power supply circuit includes a load, a diode, and an inductor; theload, the diode and the inductor are connected in series to form a ringcircuit, one end of the load power supply circuit is connected to anoperating voltage input terminal, the other end of the load power supplycircuit is connected to a first terminal of the transistor switch and afirst end of the voltage-diving resistor, a second end of thevoltage-dividing resistor and a second end of the transistor switch areconnected to the ground; the load voltage is controlled by thetransistor switch, the voltage-dividing terminal of the voltage-dividingresistor is connected to the signal input terminal of the zero-crossingdetection circuit, and the zero-crossing detection circuit is used todetermine whether the diode current crosses zero.

In an alternative embodiment of the present disclosure, thezero-crossing detection circuit includes a signal input terminal, asample and maintaining module, a second reference voltage module, and asecond comparator, the sample and maintaining module is connected to thesecond reference voltage module, one end of the sample and maintainingmodule is connected to the signal input terminal; one end of the secondreference voltage module is connected to the first input terminal of thesecond comparator, and the signal input terminal is used as the secondinput terminal of the second comparator.

In an alternative embodiment of the present disclosure, in the loadpower supply circuit, one end of the load, a cathode of the diode isconnected to the operating voltage input terminal, and the other end ofthe load is connected to one end of the inductor; the other end of theinductor is connected to the anode of the diode and is connected to afirst end of the transistor switch and a first end of thevoltage-dividing resistor.

In an alternative embodiment of the present disclosure, the load powersupply circuit includes: a transformer, a diode, and a load, one end ofthe load is connected to the cathode of the diode, the other end of theload is connected to one end of a secondary winding of the transformer,the other end of the secondary winding of the transformer is connectedto the anode of the diode, one end of the primary winding of thetransformer is connected to the operating voltage input terminal, andthe other end of the primary winding of the transformer is connected tothe first end of the transistor switch and the first end of thevoltage-dividing resistor.

In an alternative embodiment of the present disclosure, in the loadpower supply circuit, one end of the load, one end of the inductor areconnected to the operating voltage input terminal; the other end of theload is connected to the cathode of the diode, and the anode of thediode is connected to the other end of the inductor, and is connected tothe first end of the transistor switch and the first end of thevoltage-dividing resistor.

In an alternative embodiment of the present disclosure, the load is anLED.

In an alternative embodiment of the present disclosure, the load isconnected in parallel with a second capacitor.

Embodiments of the present disclosure further provide a method fordetecting current zero-crossing point corresponding to the circuit fordetecting current zero-crossing point, including

obtaining a voltage waveform at a voltage-dividing terminal of thevoltage-dividing resistor using the voltage-dividing resistor;

when the transistor switch is turned off and the diode is turned on,sampling and maintaining the voltage of the voltage-dividing terminal ofthe voltage-dividing resistor, comparing a result obtained bysubtracting the voltage by a reference voltage with the voltage at thevoltage-dividing terminal of the voltage-dividing resistor obtained inreal time; when an instantaneous voltage of the voltage of thevoltage-dividing terminal obtained in real time is lower than a resultobtained by subtracting the voltage at the voltage-dividing terminal bythe reference voltage, determining the current zero-crossing point ofthe diode.

In an alternative embodiment, the reference voltage is provided by thesecond reference voltage module.

Embodiments of the present disclosure further provide a circuit fordetecting load voltage, wherein it comprises: a load power supplycircuit, a voltage-dividing resistor, a transistor switch, a voltagedetection circuit, and zero-crossing detection circuit; the load powersupply circuit comprise a load, a diode, and an inductor, and the load,the diode and the inductor are connected in series to form a ringcircuit; one end of the load power supply circuit is connected to anoperating voltage input terminal, and the other end of the load powersupply circuit is connected to a first end of the transistor switch anda first end of the voltage-dividing resistor; the second end of thevoltage-dividing resistor and a second end of the transistor switch areconnected to the ground; the load voltage is controlled by thetransistor switch, and the voltage-dividing terminal of thevoltage-dividing resistor is connected to a signal input terminal of thevoltage detection circuit and the signal input terminal of thezero-crossing detection circuit, the zero-crossing detection circuit isused to determine whether the diode current crosses zero and to obtainthe on time of the diode, the voltage detection circuit obtains the loadvoltage by use of the voltage sampled and maintained at thevoltage-dividing terminal, the on time of the transistor switch and theon time of the diode.

In an alternative embodiment of the present disclosure, the voltagedetection circuit includes a first signal input terminal, a secondsignal input terminal, a sampling and maintaining module, a voltageselection module and an average module; the first signal input terminalis connected to one end of the sampling and maintaining module, tosample and maintain the voltage at the voltage-dividing terminal, theother end of the sampling and maintaining module is connected to a firstend of the voltage selection module; a second end of the voltageselection module is connected to the ground, and the second signal inputterminal is connected to the voltage selection module for receiving theon time of the diode and on time of the transistor switch; a selectionterminal of the voltage selection module is connected to one end of theaverage module; the average module averages the input signals during theon time of the transistor switch and the on time of the diode, and anaverage value output is the load voltage.

In an alternative embodiment of the present disclosure, thezero-crossing detection circuit comprises a signal input terminal, asampling and maintaining module, a second reference voltage module and asecond comparator; the sampling and maintaining module is connected tothe second reference voltage module, and one end of the sampling andmaintaining module is connected to the signal input terminal; one end ofthe second reference voltage module is used as a first input terminal ofthe second comparator, and the signal input terminal is used as thesecond input terminal of the second comparator.

In an alternative embodiment, the circuit for detecting load voltage andthe zero-crossing detection circuit share the sampling and maintainingmodule.

In an alternative embodiment of the present disclosure, the voltagedetection circuit and the zero-crossing detection circuit are located inthe load control circuit; when overvoltage of the load voltage isdetected, the load control circuit controls a control terminal of thetransistor switch to turn off the transistor switch.

In an alternative embodiment of the present disclosure, a first end ofthe transistor switch is connected to the other end of the load powersupply circuit; a second end of the transistor switch is connected tothe ground, and the control terminal of the transistor switch isconnected to the load control circuit.

In an alternative embodiment of the present disclosure, in the loadpower supply circuit, one end of the load, a cathode of the diode areconnected to the operating voltage input terminal; the other end of theload is connected to one end of the inductor, and the other end of theinductor is connected to an anode of the diode and is connected to thefirst end of the transistor switch and the first end of thevoltage-dividing resistor.

In an alternative embodiment of the present disclosure, the load powersupply circuit includes: a transformer, a diode, and a load; one end ofthe load is connected to the cathode of the diode, and the other end ofthe load is connected to one end of a secondary winding of thetransformer; the other end of the secondary winding of the transformeris connected to the anode of the diode; one end of the primary windingof the transformer is connected to the operating voltage input terminal,and the other end of the primary winding of the transformer is connectedto a first end of the transistor switch and a first end of thevoltage-dividing resistor.

In an alternative embodiment of the present disclosure, in the loadpower supply circuit, one end of the load and one end of the inductorare connected to the operating voltage input terminal, and the other endof the load is connected to the cathode of the diode, and the anode ofthe diode is connected to the other end of the inductor, and isconnected to the first end of the transistor switch and the first end ofthe voltage-dividing resistor.

In an alternative embodiment of the present disclosure, the load is anLED.

In an alternative embodiment of the present disclosure, the load isconnected to the second capacitor in parallel.

Embodiments of the present disclosure further provide a method fordetecting load voltage using the circuit for detecting load voltage,wherein it includes:

sampling voltage at a voltage-dividing terminal of a voltage-dividingresistor by using the voltage-dividing resistor;

when the transistor switch is turned off and the diode is turned on,sampling and maintaining the voltage of the voltage-dividing terminal ofthe voltage-dividing resistor; comparing a result obtained bysubtracting the voltage by a reference voltage with a voltage of thevoltage-dividing terminal of the voltage-dividing resistor obtained inreal time, and when an instantaneous voltage of the voltage of thevoltage-dividing terminal obtained in real time is lower than the resultobtained by subtracting the voltage of the voltage-dividing voltage bythe reference voltage, determining if diode current crosses zero, so asto obtain on time of the diode;

after obtaining the on time of the diode, obtaining the load voltage byusing the voltage sampling and maintaining at the voltage-dividingterminal, the on time of the transistor switch and the on time of thediode.

In an alternative embodiments of the present disclosure, during the ontime of the transistor switch, the voltage sampled and maintained at thevoltage-dividing terminal in the previous period is sampled when thediode is turned on, and during the on time of the diode, 0V voltage issampled, the sampling voltages during the two times periods is averaged,and the average value is the load voltage.

Compared with the prior art, the technical solutions have the followingadvantages.

In the technical solutions of the present disclosure, aftervoltage-dividing by the voltage-dividing resistor, the currentzero-crossing point of the diode can be easily detected by thezero-crossing detection circuit so as to obtain the on time of thediode. The detection efficiency is high, and the cost is low.

Meanwhile, the present technical solutions firstly use the zero-crossingdetection circuit to detect the current zero-crossing point of the diodeand obtain the on time of the diode, then use the circuit for detectingload voltage to calculate the load voltage by use of the sampling andmaintaining voltage of the voltage-dividing terminal, the on time of thetransistor switch and the on time of the diode. The circuits have simplestructures but with high detection efficiency and low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural schematic view of the circuit for detecting loadvoltage according to a first embodiment of the present disclosure.

FIG. 2 is a structural schematic view of the zero-crossing detectioncircuit according to the embodiment of the present disclosure;

FIG. 3 is a structural schematic view of the voltage detection circuitaccording to the embodiment of the present disclosure;

FIGS. 4 and 5 are the voltage and current waveform view of theembodiment according to the present disclosure;

FIG. 6 is a structural schematic view of the circuit for detecting loadvoltage according to a second embodiment of the present disclosure;

FIG. 7 is a structural schematic view of the circuit for detecting loadvoltage according to a third embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

The following will describe the technical solutions of the presentdisclosure clearly and completely by specific embodiments by combiningthe accompanying drawings.

Refer to FIG. 1, it is a circuit for detecting load voltage according toan embodiment of the present disclosure, including: a load power supplycircuit 14, a voltage-dividing resistor 16, a transistor switch 15, avoltage detection circuit 18, and a zero-crossing detection 19; the loadpower supply circuit 14 includes a load 11, a diode 13, and an inductor12, and the load 11, the diode 13 and the inductor 12 are connected inseries to form a ring circuit; one end of the load power supply circuit14 is connected to an operating voltage input terminal Vin, and theother end of the load power supply circuit 14 is connected to a firstend of the transistor switch 15 and a first end of the voltage-dividingresistor 16; a second end of the voltage-dividing resistor 16 and asecond end of the transistor switch 15 are connected to the ground, andthe load voltage is controlled by the transistor switch 15; thevoltage-dividing end of the voltage-dividing resistor 16 is connected tothe signal input terminal of the voltage detection circuit 18 and thesignal input terminal of the zero-crossing detection circuit 19. Whetherthe current of the diode crosses zero determined by the zero-crossingdetection circuit 19, and the on time of the diode is obtained, and thevoltage detection circuit 18 acquires feedback voltage of thevoltage-dividing terminal of the voltage-dividing resistor 16 bysampling and maintaining, and obtains the load voltage by calculatingusing the on time of the transistor switch and the on time of the diode.

In this embodiment, the load power supply circuit 14 includes a diode13, an inductor 12, and a load 11; one end of load 11, a cathode ofdiode 13 are connected to the operating voltage input terminal Vin, andthe other end of load 11 is connected to one end of inductor 12; theother end of inductor 12 is connected to the anode of diode 13, and isconnected to a first end of transistor switch 15 and a first end of thevoltage-dividing resistor 16.

In this embodiment, load 11 is an LED. Moreover, the directions of diode13 and the LED are identical.

In other embodiments, the load may be other electronic devices, and thevoltage of the load is detected by use of the voltage detection circuit.

A first capacitor C1 is connected to between the operating voltage inputterminal Vin and the ground, and the first capacitor is to keep thestability of the operating voltage. In other embodiments, the two endsof the LED are further connected to a second capacitor, which is usedfor filtering the load current.

A first end of the transistor switch 15 is connected to the other end ofthe load power supply circuit 14, and a second end of the transistorswitch 15 is connected to the ground; a control terminal of thetransistor switch 15 is connected to the load control circuit 17.

In this embodiment, the transistor switch is an MOS transistor. In otherembodiments, the transistor switch may be triode, etc.

In this embodiment, the voltage-dividing resistor 16 includes a firstvoltage-dividing resistor R1 and a second voltage-dividing resistor R2,and the first voltage-dividing resistor R1 and the secondvoltage-dividing resistor R2 are used to divide the voltage of the firstend of transistor switch 15; the voltage-dividing end between the firstvoltage-dividing resistor R1 and the second voltage-dividing resistor R2is connected to the signal input terminal of the zero-crossing detectioncircuit 19 and the first signal input terminal of the voltage detectioncircuit 18.

In other embodiments, the voltage-dividing resistor may also be aresistor, and the voltage-dividing end of the voltage-dividing resistoris connected to the signal input terminal of the zero-crossing detectioncircuit and the first signal input terminal of the voltage detectioncircuit.

In this embodiment, referring to FIG. 2, the zero-crossing detectioncircuit 19 includes a signal input terminal 91, a sampling andmaintaining module 93, a second reference voltage module 94, and asecond comparator 95; the sampling and maintaining module 93 isconnected to the second reference voltage module 94, and one end of thesampling and maintaining module 93 is connected to the signal inputterminal 91; one end of the second reference voltage module 94 is usedas the first input terminal of the second comparator 95, and the signalinput terminal 91 is used as the second input terminal of the secondcomparator 95.

After the transistor switch is turned off and the diode is turned on,the sampling and maintaining module 93 samples and maintains feedbackvoltage V_(FB) of the voltage terminal obtained at the signal inputterminal 91, to obtain the sampling and maintaining voltage V_(FBS), andby voltage V1 of the second reference voltage module 94, making thevoltage of the first input terminal of the second comparator 95 beV_(FBS)−V1, and compares V_(FBS)−V1, with real-time voltage of thefeedback voltage V_(FB) of the voltage-dividing terminal. When theinstantaneous voltage of V_(FB) is lower than V_(FBS)−V1, the secondcomparator 95 is turned over, then it is determined that the diodecurrent crosses zero.

The current zero-crossing point of the diode can be easily obtained bythe zero-crossing detection circuit 19, so as to obtain the on time ofthe diode. The zero-crossing detection circuit 19 may form a circuit fordetecting current zero-crossing point independently with the load powersupply circuit 14, a voltage-dividing resistor 16, and a transistorswitch 15, to detect the current zero-crossing point of the diode.However, in this embodiment, after the zero-crossing detection circuit19 obtains the on time of the diode, the circuit for detecting loadvoltage 18 may easily get the load voltage by use of the on time of thediode and on time of the transistor switch.

In this embodiment, please referring to FIG. 3, the voltage detectioncircuit 18 includes a first signal input terminal 81, a second signalinput terminal 82, a sampling and maintaining module 83, a voltageselection module 84, and an average module 85; the first signal inputterminal 81 is connected to one end of the sampling and maintainingmodule 83, to sample and maintain the voltage at the voltage-dividingterminal of the voltage-dividing resistor; the other end of the samplingand maintaining module 83 is connected to a first terminal of thevoltage selection module 84, and a second end of the voltage selectionmodule 84 is connected to the ground, i.e., 0V; the second signal inputterminal 82 is connected to the voltage selection module 84, forreceiving the on time of the diode and on time of the transistor switch;the selection terminal of the voltage selection module 84 is connectedto one end of the average module 85, and the average module 85 averagesthe input signals during the on time of the transistor switch and ontime of the diode. During the on time of the transistor switch, theselection terminal of the voltage selection module 84 is connected to afirst end of the voltage selection module 84, to average the voltagesampled and maintained at the voltage-dividing terminal; during the ontime of the diode, the selection terminal of the voltage selectionmodule 84 is connected to a second end of the voltage selection module84, to average 0V voltage, and the output average value is the loadvoltage.

When the circuit operates in a continuous current mode or a criticalcontinuous current mode, wherein the continuous current mode refers toduring the freewheeling period of the diode, when the inductor currentreduces and not yet reaches zero, the next switch period starts, so theinductor current is always kept as a positive value. The criticalcontinuous current mode refers to during the freewheeling period of thediode, when the inductor current falls to zero, the next switch periodimmediately starts, so the inductor current has a point of zero in eachperiod. The waveform of the feedback voltage V_(FB) at thevoltage-dividing terminal is as shown in FIG. 4, wherein, K=R2/(R1+R2).When the transistor switch is turned on, V_(FB)=0; when the transistorswitch is turned off and the inductor current is a positive value, thediode is turned on, the on voltage reduction of the diode is omitted,and V_(FB)=K*Vin. The on time ratio of the transistor switch is D, andthe on time ratio of the diode is D′=(1−D).

When the circuit operates in the discontinued current mode, thediscontinued current mode refers to that during the freewheeling periodof the diode, the inductor current is reduced to zero, and after acertain time period, the next switch period starts, so the inductorcurrent is zero for a time period in each period, and the waveform ofthe feedback voltage V_(FB) of the voltage-diving terminal is as shownin FIG. 5. When the transistor switch is turned off, and the inductorcurrent is a positive value, the diode is turned on, and the on voltagereduction of the diode is omitted, wherein V_(FB)=K*Vin; during the ontime of the diode, the inductor current reduces gradually, and when theinduction current is falls to zero, the diode is automatically turnedoff, and the voltage of the first end of the transistor switch isreduced, the feedback voltage V_(FB) of the voltage-diving terminal andthe voltage of the first end of the transistor switch reduced inproportion, and the feedback voltage V_(FB) of the voltage-dividingterminal is lower than the voltage when the diode is turned on, it isdetermined that the inductor current crosses zero.

In this embodiment, the sample and maintaining module 83 samples andmaintains the voltage of the voltage-dividing terminal when the diode isturned on in the previous period; during the on time of the transistorswitch, the selection terminal of the voltage selection module 84 isconnected to a first end of the voltage selection module 84, to averagethe voltage sampled and maintained at the voltage-dividing terminal, andduring the on time of the diode, the selection terminal of the voltageselection module 84 is connected to a second end of the voltageselection module 84, to average 0V voltage, corresponding toreconstructing a dot line waveform of V_(FBS) according to the feedbackvoltage V_(FB) of the voltage-dividing terminal in FIG. 5 (referring toFIG. 5). When the transistor switch 15 is turned on, the dot linewaveform voltage is the feedback voltage V_(FB) of the voltage-dividingterminal when the diode is turned on in the previous period, i.e.,K*Vin. In other time, the dot line waveform voltage is connected to theground, and is zero. The dot line waveform is averaged during D and D′(D is on time ratio of the transistor switch, and D′ is on time ratio ofthe diode), thus LED voltage is obtained, K*V_(LED)=K*Vin*D/(D+D′). Forthe continuing current mode or the critical continuing current mode,D′=(1−D); for the discontinuing current mode, D′<(1−D), during the timewhen the transistor switch and the diode are not turned on, the averagemodule does not perform the averaging.

In this embodiment, the zero-crossing detection circuit 19 and thevoltage detection circuit 18 are in the same load control circuit 17 ora control chip, and the load control circuit 17 or the control chip isused for obtaining the on time of the diode and the on time of thetransistor switch; meanwhile, when overvoltage of the load voltage isdetected, the transistor switch 15 is turned off by the control terminalVc of the transistor switch.

The voltage detection circuit 18 and the zero-crossing detection circuit19 both use the same sampling and maintaining module or use differentsampling and maintaining module.

In this embodiment, the control terminal of the zero-crossing detectioncircuit 19, the voltage detection circuit 18 and the transistor switch15 may also be in different control chips or the load control circuits.

The second embodiment of the present disclosure provides another circuitfor detecting load voltage, referring to FIG. 6, the other parts are thesame, only the load power supply circuit is different. The load powersupply circuit 20 includes: a transformer 22, a diode 23, a secondcapacitor 24, and a load 21. The second capacitor 24 is connected to theload 21 in parallel, one end of the second capacitor 24 is connected tothe cathode of the diode 23, and the other end of the second capacitor24 is connected to one end of the secondary winding of the transformer22, the other end of the secondary winding of the transformer 22 isconnected with the anode of the diode 23, one end of the primary windingof the transformer 22 is connected to the operating voltage inputterminal Vin, and the other end of the primary winding of thetransformer 22 is connected to a first end of the transistor switch 15and a first end of the voltage-dividing resistor 16.

The on time of transistor switch 15 is D, and the on time of diode 22 isD′, the relationship between the two satisfies D*Vin=D′*n*Vo, Vo is theoutput voltage, i.e., LED voltage, n is a transformation ratio oftransformer 22, the waveform at the first end of the transistor switchis reconstructed as voltage D (Vin+n*Vo), that is, the voltage of thefirst end of the transistor switch when the diode is turned on, andvoltage is zero during period D′, the voltage is averaged in period(D+D′) to obtain D*(Vin+n*Vo)/(D+D′)=n*Vo, i.e., the output loadvoltage.

In other embodiments, the two ends of the load may not be connected tothe second capacitor in parallel.

The third embodiment of the present disclosure further provides anothercircuit for detecting load voltage. Please referring to FIG. 7, theother parts are the same, and merely the load power supply circuits aredifferent. The load power supply circuit 30 includes: a diode 33, aninductor 32, a second capacitor 34, and a load 31. The load 31 isconnected to the second capacitor 34 in parallel, One ene of the load 31and one end of the inductor 32 are connected to the operating voltageinput terminal Vin, and the other end of the load 31 is connected to acathode of diode 33. The anode of diode 33 is connected to the other endof inductor 32, and is connected to the first end of the transistorswitch 15 and the first end of the voltage-dividing resistor 16.

The on time of the transistor switch 15 is D, and the on time of diode22 is D′, the relationship between the two satisfies D*Vin=D′*Vo; thewaveform of the first end of the transistor switch 15 is reconstructedas the voltage (Vin+Vo), that is, the voltage of the first end of thetransistor switch when the diode is turned on, and voltage at period D′is 0; the voltage is averaged in period (D+D′) to obtainD*(Vin+Vo)/(D+D′)=Vo, i.e., the output load voltage.

In other embodiments, the two end of the load may not be connected tothe second capacitor in parallel.

Based on the above circuit, embodiments of the present disclosure firstprovide a method for detecting current zero-crossing point, including:

-   -   obtaining a voltage at a voltage-dividing terminal of a        voltage-dividing resistor by the voltage-dividing resistor;    -   when the transistor switch is turned off and the diode is turned        on, sampling and maintaining the voltage at the voltage-dividing        terminal of the voltage-dividing resistor, comparing a result        obtained by subtracting the voltage by a reference voltage with        the voltage at the voltage-dividing terminal of the        voltage-dividing resistor obtained in real time, and when an        instantaneous voltage of the voltage-dividing terminal obtained        in real time is lower than the result obtained by subtracting        the voltage at the voltage-dividing terminal by a reference        voltage, determining the diode current crosses zero and then the        on time of the diode is obtained.

In addition, based on the above circuit for detecting load voltage,embodiments of the present disclosure further provide a method fordetecting load voltage, comprising:

-   -   sampling voltage at the voltage-dividing terminal of the        voltage-dividing resistor at the voltage-dividing resistor;    -   when the transistor switch is turned off and the diode is turned        on, sampling and maintaining the voltage at the voltage-dividing        terminal of the voltage-dividing resistor, comparing a result        obtained by subtracting the voltage by a reference voltage with        the voltage at the voltage-dividing terminal of the        voltage-dividing resistor obtained in real time, when the        instantaneous voltage of the voltage-dividing terminal obtained        in real time is lower than the result obtained by subtracting        the voltage at the voltage-dividing terminal by a reference        voltage, determining the diode current crosses zero, so as to        obtain the on time of the diode;

After the diode on time is obtained, using the sampling and maintainingvoltage at the voltage-dividing terminal, the on time of the transistorswitch and the on time of the diode, to calculate and obtain the loadvoltage.

In the first embodiment of the present invention, when the transistorswitch 15 is turned on, the sampling and maintaining dot line waveformvoltage is the feedback voltage V_(FB) at the voltage terminal when thediode is turned on in the previous period, i.e., K*Vin. When the diodeis turned on, the voltage selection module connects the output voltageto the ground, and the output voltage is adjusted to 0V, and the dotline waveform is averaged during D and D′ (D is on time ratio of thetransistor switch, and D′ is on time ratio of the diode), and the LEDvoltage can be obtained, i.e., K*V_(LED)=K*Vin*D/(D+D′).

In the second embodiment of the present disclosure, the on time oftransistor switch 15 is D, and on time of diode 22 is D′, therelationship between the two satisfies D*Vin=D′*n*Vo, and the waveformof the first end of the transistor switch is reconstructed as voltage(Vin+n*Vo), i.e., the voltage at the first end of the transistor switchwhen the diode is turned on during period D, and voltage is 0 at periodD′; the voltage is averaged within time (D+D′) to obtainD*(Vin+n*Vo)/(D+D′)=n*Vo, i.e., the output load voltage.

In the third embodiment of the present disclosure, the on time oftransistor switch 15 is D, the on time of diode 22 is D′, and therelationship between the two satisfy D*Vin=D′*Vo; the waveform of thefirst terminal of the transistor switch is reconstructed to be voltage(Vin+Vo), that is, the voltage of the first end of the transistor switchwhen the diode is turned on during period D, and voltage is 0 at periodD′; the voltage is averaged in time (D+D′) to get D*(Vin+Vo)/(D+D′)=Vo,i.e., the output load voltage.

Though the present disclosure has been disclosed above by preferredembodiments, they are not intending to limit the present disclosure. Anyof those skilled workers in the art may use the above disclosed methodsand technical contents to make possible changes and modifications on thetechnical solution of the present invention without departing from thesprints and scope of the present disclosure. Therefore, any contents notdeparting from the technical solutions of the present disclosure, anysimple amendments, equivalent changes and modifications made to theabove embodiments according to the technical substances of the presentdisclosure, all belong to the protection scope of the technical solutionof the present disclosure.

The invention claimed is:
 1. A circuit for detecting a currentzero-crossing point, comprising: a load power supply circuit; avoltage-dividing resistor; a transistor switch; and a zero-crossingdetection circuit, wherein said load power supply circuit comprises aload, a diode, and a transformer; said load, said diode and a secondarywinding of said transformer are connected in series to form a ringcircuit, one end of a primary winding of said transformer is coupled toan operating voltage input terminal, the other end of said primarywinding of said transformer is coupled to a first terminal of saidtransistor switch and a first end of said voltage-dividing resistor, asecond end of said voltage-dividing resistor and a second end of saidtransistor switch are coupled to ground; a load voltage is controlled bysaid transistor switch, a voltage-dividing terminal of saidvoltage-dividing resistor is coupled to a signal input terminal of saidzero-crossing detection circuit, and a current zero-crossing point ofsaid diode is obtained by said zero-crossing detection circuit, whereinby said zero-crossing detection circuit, a first voltage at saidvoltage-dividing terminal is sampled and maintained when said transistorswitch is turned off and said diode is turned on, a result obtained bysubtracting said first voltage by a reference voltage is compared with asecond voltage at said voltage-dividing terminal obtained in real time,to determine that a diode current crosses zero in a case that saidsecond voltage is smaller than said first voltage, wherein saidvoltage-dividing resistor is configured to divide voltage of said firstterminal of said transistor switch and provide a divided voltage at saidvoltage-dividing terminal.
 2. The circuit for detecting the currentzero-crossing point according to claim 1, wherein said zero-crossingdetection circuit comprises a signal input terminal, a sample andmaintaining module, a second reference voltage module, and a secondcomparator, wherein said sample and maintaining module is coupled tosaid second reference voltage module, one end of said sampling andmaintaining module is coupled to said signal input terminal; one end ofsaid second reference voltage module is used as a first input terminalof said second comparator, and said signal input terminal is used as asecond input terminal of said second comparator.
 3. The circuit fordetecting the current zero-crossing point according to claim 1, whereinone end of said load is coupled to said cathode of said diode, the otherend of said load is coupled to one end of said secondary winding of saidtransformer, the other end of said secondary winding of said transformeris coupled to an anode of said diode.
 4. The circuit for detecting thecurrent zero-crossing point according to claim 1, wherein said load isan LED.
 5. The circuit for detecting the current zero-crossing pointaccording to claim 1, wherein said load is connected in parallel with asecond capacitor.
 6. A method for detecting a current zero-crossingpoint by use of the circuit for detecting the current zero-crossingpoint according to claim 1, comprising: obtaining a first voltage atsaid voltage-dividing terminal of said voltage-dividing resistor;sampling and maintaining said first voltage at said voltage-dividingterminal of said voltage-dividing resistor when said transistor switchis turned off and said diode is turned on; comparing a result obtainedby subtracting said first voltage by a reference voltage with a secondvoltage at said voltage-dividing terminal of said voltage-dividingresistor obtained in real time; and determining that a diode currentcrosses zero in a case that said second voltage is smaller than saidfirst voltage.
 7. The method for detecting the current zero-crossingpoint according to claim 6, wherein said reference voltage is providedby a second reference voltage module.
 8. A circuit for detecting a loadvoltage, comprising: a load power supply circuit; a voltage-dividingresistor; a transistor switch; a voltage detection circuit; and azero-crossing detection circuit, wherein said load power supply circuitcomprises a load, a diode, and a transformer, and said load, said diodeand a secondary winding of said transformer are connected in series toform a ring circuit; one end of a primary winding of said transformer iscoupled to an operating voltage input terminal, and the other end ofsaid primary winding of said transformer is coupled to a first end ofsaid transistor switch and a first end of a voltage-dividing resistor; asecond end of said voltage-dividing resistor and a second end of saidtransistor switch are coupled to ground; a load voltage is controlled bysaid transistor switch, and a voltage-dividing terminal of saidvoltage-dividing resistor is coupled to a signal input terminal of saidvoltage detection circuit and to a signal input terminal of saidzero-crossing detection circuit, said zero-crossing detection circuit isused to determine whether a diode current crosses zero and to obtain anon time of said diode; said voltage detection circuit obtains said loadvoltage by use of a first voltage sampled and maintained at saidvoltage-dividing terminal, said on time of said transistor switch andsaid on time of said diode, wherein by said zero-crossing detectioncircuit, a first voltage at said voltage-dividing terminal is sampledand maintained when said transistor switch is turned off and said diodeis turned on, a result obtained by subtracting said first voltage by areference voltage is compared with a second voltage at saidvoltage-dividing terminal obtained in real time, to determine that adiode current crosses zero in a case that said second voltage is smallerthan said first voltage, wherein said voltage-dividing resistor isconfigured to divide voltage of said first terminal of said transistorswitch and provide a divided voltage at said voltage-dividing terminal.9. The circuit for detecting the load voltage according to claim 8,wherein said voltage detection circuit comprises a first signal inputterminal, a second signal input terminal, a sampling and maintainingmodule, a voltage selection module and an average module; said firstsignal input terminal is coupled to one end of said sampling andmaintaining module, to sample and maintain said voltage at saidvoltage-dividing terminal, the other end of said sampling andmaintaining module is coupled to a first end of said voltage selectionmodule; a second end of said voltage selection module is coupled toground, and said second signal input terminal is coupled to said voltageselection module for receiving said on time of said diode and said ontime of said transistor switch; a selection terminal of said voltageselection module is coupled to one end of said average module; saidaverage module averages said input signals when said transistor switchis turned on and said diode is turned on, and outputs an average valueas said load voltage.
 10. The circuit for detecting the load voltageaccording to claim 8, wherein said zero-crossing detection circuitcomprises a signal input terminal, a sampling and maintaining module, asecond reference voltage module and a second comparator, wherein saidsampling and maintaining module is coupled to said second referencevoltage module, and one end of said sampling and maintaining module iscoupled to said signal input terminal; one end of said second referencevoltage module is used as a first input terminal of said secondcomparator, and said signal input terminal is used as said second inputterminal of said second comparator.
 11. The circuit for detecting theload voltage according to claim 8, wherein said circuit for detectingload voltage and said zero-crossing detection circuit share a samplingand maintaining module.
 12. The circuit for detecting the load voltageaccording to claim 8, wherein said voltage detection circuit and saidzero-crossing detection circuit are located in a load control circuit;when overvoltage of said load voltage is detected, said load controlcircuit controls a control terminal of said transistor switch to turnoff said transistor switch.
 13. The circuit for detecting the loadvoltage according to claim 8, wherein said control terminal of saidtransistor switch is coupled to said load control circuit.
 14. Thecircuit for detecting the load voltage according to claim 8, wherein oneend of said load is coupled to a cathode of said diode, and the otherend of said load is coupled to one end of said secondary winding of saidtransformer; the other end of said secondary winding of said transformeris coupled to an anode of said diode.
 15. The circuit for detecting theload voltage according to claim 8, wherein said load is an LED.
 16. Thecircuit for detecting the load voltage according to claim 8, whereinsaid load is connected to said second capacitor in parallel.